Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) and convert received acoustic waves to electrical signals via inverse and direct piezoelectric effect. Acoustic transducers generally include acoustic resonators, such as surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, BAW resonators include thin film bulk acoustic resonators (FBARs), which may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may form a thin membrane.
FBAR devices, in particular, generate longitudinal acoustic waves and lateral acoustic waves when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The longitudinal acoustic wave, usually called a piston mode, is electrically excited by a vertical electric field between electrode plates and has a form of laterally uniform motion with the boundaries of motion determined by an overlap of top and bottom electrodes and the piezoelectric material. Lateral acoustic waves, usually called lateral modes, are excited at the edges of the piston mode motion and facilitate continuity of appropriate mechanical displacements and stresses between electrically excited and non-excited regions. In general, lateral modes are specific forms of motion supported by a mechanical stack and have both longitudinal and shear components. The lateral modes can either propagate freely (so called propagating modes) or exponentially decay (so called evanescent and complex modes) from the point of excitation. These modes can be excited both by a lateral mechanical discontinuity (for example, at an interface between a frame and a membrane, or at the edge of a top or bottom electrode) or by electric field discontinuity (for example, at an edge of a top electrode where the electric field is terminated abruptly). The lateral modes and the higher order harmonic mixing products generally have a deleterious impact on functionality.
In certain configurations, a frame may be provided along one or more sides of an FBAR to mitigate acoustic losses at the boundaries by minimizing scattering of electrically excited piston mode at the top electrode edges and by improving confinement of mechanical motion to the active region of the FBAR (the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode). In general, frames are made of added (or removed) thin layers of material along the perimeter of the resonator device with the purpose of lowering (increasing) the cutoff frequency in that region with respect to the main membrane. This in turn minimizes the amplitude of the electrically excited piston mode and the resulting scattering at top electrode edges above (or below) the cut-off frequency of a membrane. Frames also create an acoustic impedance mismatch that enables suppression of the amplitudes of propagating and/or evanescent modes (whichever exist in the frequency range of interest) mechanically excited at the membrane/frame boundary, thus further minimizing acoustic energy leakage to the outside of the active region. However, in addition to improved acoustic energy confinement, as well as further improvements in FBAR quality factor Q due to the better acoustic energy confinement, simplified design and implementation of frames are needed. In particular, in some applications, frames placed above the piezoelectric layer are not effective in suppressing modes confined to the bottom part of the stack.
In addition, FBAR filters in particular need to guarantee sufficiently low insertion loss (IL) across temperature ranges, as well as frequency ranges. Typically, as ambient temperature increases, sound velocity of most materials decreases and the cutoff frequency of each of the FBARS forming the filter decreases. Thus, as the temperature increases, the pass-band of the filter generally moves towards lower frequencies. Therefore, in the absence of temperature compensation, the pass-band must be designed wide enough to allow for changes of the ambient temperature, requiring high a coupling coefficient kt2 of each FBAR, which may be difficult to achieve. Also, in some cases (e.g., Band 13), the pass-band may not be allowed to move to prevent encroachment on other (e.g. safety) bands. Temperature compensation of the filter (and therefore each FBAR) is sometimes required. For example, boron-doped silicon oxide SiOx (which may be referred to as “tempco oxide”) may be added as a temperature compensating layer to the FBAR. The sound velocity of tempco oxide increases with temperature, which yields the desired stabilization of resonator and filter response with changes in ambient temperature. The temperature compensating layer may be embedded into either top or bottom electrode, with all the associated process complications. Typical structures to improve Rp and Q are then used: top electrode air-bridges (to eliminate dead-FBAR) and add-on frames on top electrode (to minimize scattering at the top electrode edges.
Typically, the temperature compensating layer lowers the effectiveness of add-on frames used for quality factor Q improvement. The reason is that low acoustic impedance of the temperature compensating layer confines a significant amount of energy both from the piston mode and from stack eigenmodes that are confined to the part of the resonator stack where the temperature compensating layer is placed. The typical add-on frames are placed on the top of the stack in order to facilitate growth of high-quality planar piezoelectric layer. The temperature compensating layer may be placed either below or above the piezoelectric layer, which limits the effectiveness of top add-on frames on suppressing the eigen-modes confined to the bottom of the resonator stack. Thus, approaches allowing for construction of both planarized frames (that may be placed at arbitrary location in the resonator stack) and temperature compensating layers within a BAW resonator stack are needed for applications requiring high-quality factor Q and temperature compensated frequency response.